The most efficient insulation used in buildings and appliances is closed cell foam polyurethane, polyisocyanerate or phenolic insulation. When new, this insulation has over twice the insulating value per inch of thickness as does fiberglass insulation. Substantially, all commercial refrigerators made today in the U.S. use closed cell foam for the surfaces; save the doors. About seven million refrigerators are sold in the U.S. every year. One-third of the interior volume of a refrigerator is foam insulation. Industry representatives estimate that reducing insulation volume by one cubic foot would be worth $50.
About one-half of the buildings constructed in the U.S. use closed cell foam for roof or wall insulation. Use of boards made of foam as exterior sheathing is the most practical way 2.times.4 framed construction can meet the current energy standards.
Closed cell foam has excellent insulating characteristics because it contains chlorofluorocarbon (CFC) vapor within the cells. The CFC vapor has a thermal conductivity which is one-third that of air. Unfortunately, it has been found that this CFC causes depletion of the stratospheric ozone layer. The U.S. and other developed nations have agreed to phase out the use of CFC over the next decade. Replacement refrigerant vapors are not available at present. Replacements being considered have not completed toxicity tests, have higher projected costs and higher thermal conductivity, and there is little manufacturing capability in place for these replacements.
All of the above factors favor the development of alternate insulations which are inexpensive and have insulating values per unit thickness equal to or exceeding that of present day closed cell foams. One such concept is vacuum thermal insulation. Vacuum insulations have been used for thermos-type containers and for vessels containing cryogenic fluids. These are vacuums made with rigid wall containers which have a high reflectivity surface to reduce radiation heat transfer. These insulations are expensive and fragile.
Recent interest has centered on vacuum panels made with flexible surfaces. In Uekado et al., U.S. Pat No. 4,668,555, the panel is formed of evacuated foam insulation within a metal-plastics laminate film envelope. In another system, the panels are filled with lightly compressed blocks of fine powder. The powder is covered with a flexible plastic skin or packaging coated with thin metallic layers which prevent air diffusion. When the interior of the panel is evacuated to a modest vacuum, e.g., one mm Hg pressure, the effective conductivity of the powder is found to have one-third to one-fifth the conductivity of the best closed cell foam insulation. The powder also supports the skin so that it does not collapse when it is evacuated.